Sorry - not true. The range of ships used by privateers was quite large. They did not appear to include ships of the line, but did include vessels up to the size of navy sloops and cutters. Over 1700 letters o marque were issued in the revolutionary war. http://www.nps.gov/revwar/about_the_revolution/privateers.html
After reading that, it appears that the broader interpretation was right. These ships, one with 26 guns, legally existed in private hands. The letters simply let them legitimately conduct the pirate activities they were capable of.